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    <title>Document</title>
    <script type="text/javascript">
    const users = {
    Alex: {
    email: 'alex@alex.com',
    skills: ['HTML', 'CSS', 'JavaScript'],
    age: 20,
    isLoggedIn: false,
    points: 30
  },
  Asab: {
    email: 'asab@asab.com',
    skills: ['HTML', 'CSS', 'JavaScript', 'Redux', 'MongoDB', 'Express', 'React', 'Node'],
    age: 25,
    isLoggedIn: false,
    points: 50
  },
  Brook: {
    email: 'daniel@daniel.com',
    skills: ['HTML', 'CSS', 'JavaScript', 'React', 'Redux'],
    age: 30,
    isLoggedIn: true,
    points: 50
  },
  Daniel: {
    email: 'daniel@alex.com',
    skills: ['HTML', 'CSS', 'JavaScript', 'Python'],
    age: 20,
    isLoggedIn: false,
    points: 40
  },
  John: {
    email: 'john@john.com',
    skills: ['HTML', 'CSS', 'JavaScript', 'React', 'Redux', 'Node.js'],
    age: 20,
    isLoggedIn: true,
    points: 50
  },
  Thomas: {
    email: 'thomas@thomas.com',
    skills: ['HTML', 'CSS', 'JavaScript', 'React'],
    age: 20,
    isLoggedIn: false,
    points: 40
  },
  Paul: {
    email: 'paul@paul.com',
    skills: ['HTML', 'CSS', 'JavaScript', 'MongoDB', 'Express', 'React', 'Node'],
    age: 20,
    isLoggedIn: false,
    points: 40
  }
}
// 1.统计在线人数
const uname=Object.keys(users)
var sum=0
uname.forEach(element => users[element].isLoggedIn ? sum++:sum);
console.log('在线人数'+sum);
// // 2.统计分数大于name
var sumpoint =0
uname.forEach(element => users[element].points>50?sumpoint++:sumpoint);
console.log('大于50人数为'+sumpoint+'人');
// 3.查找会MongoDB的人，并打印其名字
const arr=[]
uname.forEach(element => users[element].skills.includes('MongoDB') ? arr.push(element) : arr)
console.log('会MongoDB的人' +arr);
// 4.查找同时会：MongoDB， Express，React，和Node 的人，并打印其名字
const arr1=[]
uname.forEach(element=>users[element].skills.includes('MongoDB','Express','React','Node')?arr1.push(element):arr1)
console.log('会MongoDB， Express，React，和Node 的人'+arr1);

var collection = {
    "1275":{
        "album":"依然范特西",
        "artist":"周杰伦",
        "tracks":[
            "听妈妈的话",
            "千里之外"
        ]
},
    "2468":{
        "album":"西界",
        "artist":"林俊杰",
        "tracks":[
            "西界",
            "杀手"
        ]
},
    "3245":{
        "artist":"许嵩",
        "tracks":[ ]
    },
    "5439":{
        "album":"遇见"
    }
}

// 1.定义方法更新记录:
// function updateRecords(id,prop,value)
// 要求：
// 1).如果传入的value是一个空值，表示将该属性删除
// 2).传入的prop如果是tracks，则代表添加，其他是修改
    function updateRecords(id,prop,value){
        if (prop=='tracks') {
          collection[id][prop]=value
        }else{
          collection[id][prop]=value
        }
        if (value=='') {
          delete collection[id][prop]
        }
    }
    updateRecords(3245,'tracks','有何不可')
    updateRecords(3245,'tracks','')
    updateRecords(5439,'artist','孙燕姿')
    updateRecords(5439,'album','逆光')
    console.log(collection);
    </script>
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